I think the answer to this one lies completely in the bounce message posted.
The message was sent from a VM mail address by an e-mail client (program) and not via VM's web mail system. How I know this - read on.
The recipient's ISP bounced it back because the VM server it came from was apparently on an RBL - real time block list, as it had been reported for sending spam. 184.108.40.206 is the IP address for one of VM's SMTP (outward bound) mail servers in NW England.
The server reporting the non-delivery is one of VM's servers in NW England as well.
I have checked the SMTP server in MX toolbox and it is not listed there so I think the RBL listing has now been withdrawn and things should be back to normal. All ISP mail servers get listed from time to time, but the admins for the server constantly monitor this and deal with adverse listings.
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